1. Field of the Invention
This invention relates to a focusing method for wide-angle type zoom lenses suited to 35 mm film format cameras and has a wide range of application as it can be used even in wide-angle zoom lenses for video cameras or electronic still cameras.
2. Description of the Related Art
For the ordinary single-lens reflex camera, the wide-angle zoom lens whose angle of view is 63.degree. or more (f=35mm or less), in many cases, has its frontmost lens group made negative in refractive power, or it employs the zoom lens of the so-called "negative lead" type. In this negative lead type zoom lens, it is usual to use its first lens group for focusing purposes. With this kind of the type of zoom lens, when the angle of view is increased up to about 84.degree. (f=24 mm or less), a drawback arises that the range of variation of field curvature with focusing increased particularly on the wide-angle side. Also, setting aside the increase of the maximum angle of view, an increase of the relative aperture leads to a drawback that the range of variation of spherical aberration with focusing increases particularly on the telephoto side.
To eliminate the drawback arising from the increase of the maximum angle of view, the first lens group may be divided into two parts which are moved at different speeds from each other so that the curvature of field for an object at a short distance is corrected, as disclosed in Japanese Laid-Open Pat. application No. Sho 52-32342.
In this example, by making nearly afocal the rays of light emerging from the front part of the first lens group, it is made possible to correct the curvature of field on the wide-angle side while simultaneously suppressing the variation of spherical aberration on the telephoto side.
Though, on the optical performance, it corrects aberrations very well, this method, because, from the point of view of the driving of the focusing lens, the first lens group of large diameter has to be moved and, moreover, the speeds of the two parts of that lens group have to be differentiated from each other, gets problems that the complexity of the operating mechanism increases and a large torque is required, and that it sometimes happens that the balance in management becomes bad.
For even another range of 34.3.degree.-63.4.degree. in the angle of view, as the so-called standard zoom lens in terms of the 35 mm single-lens reflex camera there are known 2-group zoom lenses of which the first lens group counting from front is divergent and performs compensation for the shift of the image surface due to zooming, and the second lens group is convergent and performs variation of the image magnification. Further, to the end of increasing either or both of the zoom ratio and the aperture ratio, it is known to divide the second lens group into two or three parts over which the effect of variation of the image magnification is distributed, thus providing a 3-group zoom lens or 4-group zoom lens.
Recently, the proportion of AF (auto-focus) single-lens reflex cameras is increasing. In view of this trend, to speed up focusing by achieving reduction of the weight of the focusing lens is sought as a need of the times.
Another needs, say, wider wide-angle and increased relative aperture, too, are very strong. For a lens satisfying such requirements, if the first lens group is used for focusing purposes, its diameter has to be increased, which in turn makes it heavier, thus giving a large obstacle to the faster focusing by the AF mechanism.
In the meantime, calling for the reduction of the weight of the focusing lens, an example of making movable part of a multi-movable-group zoom lens is also put into practice.
However, the present state of application of this idea is limited to the range of less than 75.degree. in the angle of view and an F-number of 4 or more, that is, relatively dark zoom lenses, since it is not very suited to high relative aperture wide-angle zoom lenses.
Meanwhile, in U.S. Pat. No. 3,682,534 (Japanese Pat. Publication No. Sho 48-6813), the first lens group is divided into two negative lens members, and focusing is performed by the rear member. Japanese Pat. Publication No. Sho 61-53696 (U.S. Pat. No. 4,752,121), too, discloses focusing by part of the first lens group. However, because such a configuration has the movable lens groups for zooming preceded by the lens group for focusing which is stationary during zooming, a difficult problem arises that the total length of the entire lens system is elongated. This increase of the size of the lens makes such a focusing method not adaptable to, in particular, wide-angle lenses for AF.
Another wide-angle zoom lens is disclosed in Japanese Laid-Open Pat. application No. Sho 57-190917, comprising, from the object side, a front group of negative refractive power and a rear group of positive refractive power, thus constituting a 2-group zoom lens. Its practical example has about 3.5 in F-number. A further increase of the relative aperture results in the tendency of making it difficult to afford a hold for the back focal distance. Attempts have been made to increase the range of such a zoom lens by dividing the aforesaid second lens group into three parts of plus-minus-plus arrangement, or by employing the four-group form, as disclosed in Japanese Laid-Open Pat. application No. Sho 55-14403, U.S. Pat. No. 4,759,617 and Japanese Laid-Open Pat. application Nos. Sho 57-11315 and Sho 63-241511. These show examples of application to relatively wide-angle zoom lenses of 2.omega.=75.4.degree. at the maximum angle of view. With such a configuration left not to change, for a wide-angle large relative aperture zoom lens, there is some possibility of failure of affording a hold for the back focal distance. As an example of application to a wider-angle zoom lens, there is a product: 24-40 mm/2.8 sold from Tokiner Optics Co. Ltd. Even this zoom lens has as low a maximum angle of view as 2.omega.=84.degree.. These have to introduce the 4-group zoom type for the purpose of increasing the relative aperture of the zoom lens.